CAS-004 Security Engineering Practice Question
A security architect is evaluating a hardware security module (HSM) deployment for a certificate authority. The requirement is that private keys must never leave the HSM in plaintext, and that key operations must be auditable. During a review, the architect learns that the HSM supports key wrapping for backup. Which of the following is the MOST important control to verify?
⚠ Common exam trap
The trap here is focusing on encryption of the backup channel while overlooking that the wrapping key itself must be protected and separated from the keys it wraps.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
That the key wrapping key is stored in a separate, tamper-resistant HSM under a different administrator's control, and that wrapping operations are logged.
The strongest control is to separate the key wrapping key into an independent, tamper-resistant HSM under different administrative control and to log all wrapping operations. This enforces separation of duties and ensures that compromise of one HSM does not expose all backed-up private keys. Exportable wrapping keys, weak ciphers like ECB, or insecure transport channels all undermine the security of the backup process.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
That the private keys are backed up to an encrypted network share using the HSM's built-in FTP client for offsite storage.
Why it's wrong here
Exporting wrapped keys over FTP, even to an encrypted share, expands the attack surface and violates the principle of keeping key material within controlled boundaries. FTP is insecure for key transport, and network shares are not tamper-resistant. Backup should use secure, authenticated channels and ideally remain within the HSM ecosystem or a dedicated key management appliance.
- ✓
That the key wrapping key is stored in a separate, tamper-resistant HSM under a different administrator's control, and that wrapping operations are logged.
Why this is correct
Separating the wrapping key into a distinct HSM under dual control enforces separation of duties and prevents a single compromised HSM from exposing backed-up keys. Logging wrapping operations provides the required auditability. This design ensures that even if the primary HSM is compromised, the attacker cannot unwrap exported blobs without also compromising the second HSM.
- ✗
That the key wrapping key is stored in the same HSM and is itself exportable for disaster recovery.
Why it's wrong here
If the key wrapping key is exportable, an attacker who compromises the backup can unwrap all backed-up private keys, defeating the purpose of keeping keys inside the HSM. The wrapping key should be non-exportable and protected by the HSM's tamper-resistant boundary. Exportability introduces a single point of failure that undermines the entire key protection model.
- ✗
That the HSM uses AES-128 in ECB mode for key wrapping to maximize performance during backup operations.
Why it's wrong here
ECB mode is deterministic and reveals patterns in plaintext, making it unsuitable for key wrapping. AES-128 is weaker than AES-256 and may not meet policy requirements. The performance gain does not justify the security loss, and modern HSMs support AES Key Wrap with Padding (RFC 5649) which is both efficient and secure.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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